The invention relates to a method for the manufacture of a composite cooling element for the melt zone of a metallurgical reactor, whereby the element is manufactured by attaching ceramic lining sections to each other by copper casting and forming at the same time a copper plate equipped with cooling water channels behind the lining. The invention also relates to composite cooling elements manufactured by this method.
The refractory of reactors in pyrometallurgical processes is protected by water-cooled cooling elements so that, as a result of cooling, the heat coming to the refractory surface is transferred via the cooling element to water, whereby the wear on the lining is significantly reduced compared with a reactor which is not cooled. Reduced wear is caused by the effect of cooling, which brings about forming of so-called autogenic lining, which fixes to the surface of the heat resistant lining and which is formed from slag and other substances precipitated from the molten phases.
Conventionally, cooling elements are manufactured in three ways: primarily, elements can be manufactured by sand casting, where cooling pipes made of a highly thermal conductive material such as copper are set in a sand-formed mould, and are cooled with air or water during the casting around the pipes. The element cast around the pipes is also of highly thermal conductive material, preferably copper. This kind of manufacturing method is described in e.g. GB patent no. 1386645. One problem with this method is the uneven attachment of the piping acting as cooling channel to the cast material surrounding it. Some of the pipes may be completely free of the element cast around it and part of the pipe may be completely melted and thus fused with the element. If no metallic bond is made between the cooling pipe and the rest of the cast element around it, heat transfer will not be efficient. Again if the piping melts completely, that will prevent the flow of cooling water. Advantages of this method are the comparatively low manufacturing costs and independence of dimensions.
Another cooling element manufacturing method of the above type is to manufacture elements by sand casting, with cooling pipes made of some other material than copper. Copper is cast around the pipes on a sand bed, and then, by overheating the casting copper a good contact is achieved between the copper and the pipes. However, in general, the thermal conductivity of said pipes is only of the order of 5-10% that of pure copper. This weakens the cooling ability of the elements, especially in dynamic situations.
U.S. Pat. No. 4,382,585 describes another, much used method of manufacturing cooling elements, according to which the element is manufactured for example from rolled or forged copper plate by machining the necessary channels into it. The advantage of an element manufactured this way, is its dense, strong structure and good heat transfer from the element to a cooling medium such as water. Its disadvantages are dimensional limitations (size) and high cost.
The biggest weakness in cooling elements fabricated with the methods mentioned above, is that it is difficult to obtain a good contact at the fitting stage between the ceramic furnace lining it is meant to protect (the fireproof lining) and the element. This means that the protective effect of the cooling element on the ceramic lining is greatly dependent on the success of the fitting, and very often it is not possible to take full advantage of the cooling properties of the element.
Now a method has been developed whereby a fixed metallic contact is made between the ceramic lining of the metallurgical reactor and a copper plate behind it furnished with cooling water piping, together forming a composite cooling element. This occurs best when the sections of the ceramic lining such as fireproof burnt bricks, are joined to each other by casting molten copper between the bricks and at the same time casting a copper plate behind the surface formed by the ceramic units. The rear section copper plate is furnished with cooling water channels, preferably double channels. The invention also relates to the composite cooling element itself, with a surface section formed of ceramic bricks in between which high thermal conductivity copper is cast, and where a copper plate furnished with cooling water channels is cast at the same time behind the surface section. The essential features will become apparent in the attached patent claims.
In practice, the cooling element is formed so that copper is cast around the burnt ceramic bricks so that the ceramic brickwork is largely formed during casting and makes a good contact with the cast copper. Due to the great thermal conductivity of copper, the protective effect of copper joints on the brickwork is effective. So that heat is not transferred needlessly, the copper joints between the bricks are made as thin as possible, preferably for technical reasons 0.5-2 cm thick. If the joints are thicker, they will conduct too much heat from the furnace to cooling, needlessly increasing heat losses and operating costs. The preferable amount of copper in the surface section of the cooling element (the section going into the inside of the reactor) in ratio to the ceramic lining is maximum 30% of the surface area, i.e. the amount of joining material should not become too massive, because the aim is not to increase total heat losses but to protect the brickwork.
Burnt bricks suitable for casting are used as the ceramic lining material, i.e. brick material, as they traditionally have good properties against metallurgical melts. The copper is a grade with high electrical conductivity, preferably higher than 85% IACS, since there is a direct dependency on the electrical and thermal conductivity of copper.
While the bricks are being joined together, a copper plate in which cooling water channels are worked, is cast behind the ceramic lining. The channels are made as double-pipe channels in the rear section of the element formed by the copper plate, e.g. by drilling so that first the outer pipe is drilled, with walls profiled to increase the heat transfer surface. An inner pipe with a smaller diameter is placed inside the outer pipe, and water is fed in through this inner pipe to the element and out through the profiled outer pipe. Using profiles, such as grooves, flutes, threads or similar on the inner surface of the pipe, the heat transfer surface of the wall can be increased by as much as double compared with a smooth surface.
Channels are made in the heat transfer element so that there is a distance of maximum 0.5-1.5 times the diameter of the channel between the channels, and is therefore a fixed part of the element. If the channels are made closer together, no benefit will be obtained, since then the heat transfer surface of the back of the channels would be utilized needlessly and also the structure would be weakened. If, on the other hand, the channeling is made farther apart, maximization of the heat transfer surface will not be utilized and then the cooling capacity will be lessened.
As mentioned above, an inner pipe is placed inside each drilled pipe in the heat transfer element, through which cooling water is conveyed into the element. From the inner pipe the water flows on in the ring-like channel formed by the outer and inner pipes and out to circulation. The double-pipe structure facilitates a reduction in flow cross-sectional area so that a higher rate is achieved with a certain amount of water than if only one pipe were used. A higher flow rate has in turn a significantly positive effect on the heat transfer between the element and the water. If the heat transfer surface is optimized using conventional smooth pipes, such an increase in heat transfer surface area could not be attained since the quantities of water would be excessively great.
The heat transfer elements are joined to each other tightly by making the sides of the elements into tongues and grooves or overlapping them so that the chinks in adjacent elements form a labyrinth.